Late frosts, low yields, a short growing season and insect damage are four common problems that home gardeners would rather do without. Fortunately, the development of rowcovers has added a new dimension to the eco-gardener's management strategy. Rowcovers are available in a variety of materials, including both plastics and fabrics. They can enhance yields of some crops, while producing an early harvest on others. They may offer a few degrees of frost protection, and when properly used, they can effectively block the feeding activity of some insects.
Extending the season
The use of protective coverings to produce a miniature greenhouse effect is certainly not a new idea. A popular practice among some European growers in the early 1900's was to place glass bell jars over individual plants. It was reported in 1910 that there were over 2 million of these jars being used in the Paris vicinity alone.
Less expensive paper materials were eventually developed which replaced the bell jars, and by the mid 1950's "hot caps" or "hot tents" were being used in several areas of the United States. Although these are still in use in home gardens today, many gardeners are enthusiastic about the advantages that rowcovers have to offer.
The ideal rowcover would be one which is inexpensive, prevents air temperatures from falling below freezing at night, increases growing degree-days, and avoids excessively high temperatures and humidity that can have a negative impact on growth and flower development. If you have already done some experimenting with rowcovers on your own, you probably have already found that no such ideal material exists at this time. In fact, there are no rowcovers that can offer more than a few degrees of frost protection. Their major benefit is associated with more rapid growth by increasing daytime temperatures, not with frost protection.
Cooler rowcover materials can be selected so that the risk of damaging high temperatures is minimized, but keep in mind that cooler materials will not increase growth and yield as quickly. You may want to use a cooler tunnel or hot caps on most of your plants, and a warmer but riskier product for earliest yields on a few plants. An approximate ranking of row cover types from warmest to coolest would be:
Most plastic rowcovers are supported by wire hoops. After the crop is planted, 10 gauge hardened galvanized wire pieces (36 in. long for 4 ft. plastic, 48 in. for 5 ft. plastic) are bent in the shape of an arch and placed in the soil at 3-4 ft. intervals. Do not place hoops directly over the plants so that you can create slits for ventilation or irrigation if necessary. The height of the tunnel is determined by the height of the crop to be grown.
Floating rowcovers made of lightweight spunbonded or woven fabrics can be placed loosely over the plants without wire supports. The edges can be held down with soil or stones. These are easier to apply, and are the choice for small gardens. Fabric materials can be used again unless they have been damaged or ripped.
Proper ventilation is essential, particularly with the hottest materials. Growers drill holes in the rolls of plastic prior to application; home gardeners may do this or slit plastic after it has been rolled out. Allowing excessive heat buildup is the most costly and most common error with first-time rowcover users. As a general rule, if outside air temperatures approach 80 degrees F., additional ventilation should be considered. In years with unseasonably warm temperatures, covers do more harm than good.
Normally, rowcovers are removed from vegetable crops 3-4 weeks after transplanting, and from Junebearing strawberries at the beginning of bloom to allow bees to pollinate. You may want to allow plants to acclimate gradually by increasing the size of slits over a 3-7 day period prior to complete removal.
For insect control, rowcovers must go on before the insects are present. Also, rotate the location of crops that attract insectpests that overwinter in the soil. If you don't you may increase insect problems by trapping the insects with their food source. For row covers to be effective barriers to insects, they must be sealed at the sides and ends of the rows, (i.e., edges covered with soil) so that insects cannot crawl under them.
Specific crop responses
Heritage Red Raspberry: Rowcovers are applied after pruning plants to the ground in March, and are removed 2 months later. The response to rowcover may vary between years, but in general, the use of rowcover can help to enhance the early production of the crop. This is beneficial since the harvest of 'Heritage', a late season raspberry, is often prematurely terminated by frost.
Junebearing Strawberries: Rowcovers can enhance both early production and yield of strawberries. In gardens experiencing high winds, cold temperatures and inconsistent snowfalls, mulch the strawberries with straw in the late fall. Remove the straw after March 1, and cover the plants immediately with the rowcovers. Place rowcovers over the plants in the late winter. Remove the covers as soon as the plants begin to blossom in the spring, or the rowcovers may interfere with pollination. Use spunbonded materials; plastics may enhance earliness to the point of creating problems with late spring frost.
Rowcovers can also reduce injury by tarnished plant bug. Due to accelerated flowering, the insects are present after the susceptible stage of development.
Muskmelon: The most success using tunnels on vegetable crops has occurred with muskmelon. Ventilated clear plastic is often used. A 7-10 day jump on the season with an increase in yields can be attained. Melons are not particularly sensitive to the high temperatures which sometimes develop with the clear plastic tunnel. Remove tunnels when the plant begins to flower.(Also see vine crops)
Summer Squash: Similar to melons, it is one of the easiest and most responsive crops to grow under tunnels.(Also see vine crops)
Cucumber: More sensitive to high temperatures than melons, the cucumber can still show a very positive response to tunnels. When temperatures are cool, a clear tunnel with holes may be best, but ventilated white plastic or fabric tunnels, however, are safest.(Also see vine crops)
Pepper: Flower abortion and loss of early yields can occur under high temperatures, so adequate ventilation is critical. Early growth and early yield can be successfully obtained with ventilated white plastic.
Tomato: In general, rowcovers are not recommended for this crop. Loss of early yields due to high temperature is the major problem; although vegetative growth and total yield may be enhanced, early fruit are frequently small or absent.
Cole Crops, Lettuce, Spinach, Celery: Rowcovers increase early production with these crops. Most are well-suited to floating rowcovers.
Spinach, Beets, and Chard: Rowcovers can help provide protection from leafminers. Place over the newly seeded crop and remove at harvest (spinach) or when plants outgrow them.
Vine Crops: Rowcovers can exclude striped cucumber beetle which can vector bacterial wilt. Rowcovers need to be removed at flowering time for pollination to occur.
David Wolfe, Associate Professor, Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853
Marvin Pritts, Associate Professor, Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853
Marcia Eames-Sheavly, Extension Support Specialist, Department of Fruit and Vegetable Science, Cornell University, Ithaca, NY 14853